In the field of foundations, in order to make impermeable or structural diaphragms, it is known to use digging equipment consisting of a base machine or “carrier”, like for example a tracked crane or a drilling machine, equipped with tracks, which supports and moves an immersion digging tool equipped with hydraulic apparatuses like, for example, a cutter. The base machine is positioned on the surface of the soil, from which the digging begins, and is always kept outside of the excavation itself to support and manoeuvre the tool. Such hydraulic functions of the tool being immersed are carried out by hydraulic actuators fixed to the tool and operatively connected to the base machine through supplying hydraulic pipes. These actuators are also, along with the relative hydraulic supply pipes, immersed in the digging fluid.
The hydraulic actuators are thus subjected to an external pressure equal to the hydrostatic pressure of the stabilizing fluid and it is possible for the digging fluid, pushed by hydrostatic pressure, to penetrate into those components of the hydraulic circuit that are at lower pressure, therefore contaminating the oil of the hydraulic system despite the presence of pressure compensation devices. Such contamination or pollution, even in small percentage concentrations, drastically reduces the lubricant properties of the oil and causes series damage such as breaking or seizure of the hydraulic components of the equipment. This damage results in the loss of some hydraulic apparatuses for moving and for digging and, in the worst case scenario, makes it impossible to extract the immersed tool from the excavation. The restoration of such functionalities is particularly expensive, requiring the replacement of the components and of the oil and causing long machine down times.
An example of a known digging equipment for making deep diaphragms is shown in FIG. 1, where it is wholly indicated with reference numeral 100. The equipment 100 can be divided mainly into a base machine 2 and into one digging tool 3 supported by the base machine 2. The base machine 2 generally consists of a tracked truck 4, a tower 5 rotating with respect to the tracked truck 4 and one arm 6, generally able to tilt and hinged to the tower 5, which supports the digging tool 3 through a suspending flexible element 7 that can be wound or unwound through a winch 8. The base machine 2 is positioned on the surface of the ground, from which the digging begins, always stays outside of the excavation itself. The base machine 2 has the task of maneuvering the digging tool 3, positioning it on the digging site, and providing such a digging tool 3 with the power needed to dig the soil.
The base machine 2 also performs multiple service functions, of which the following are essential: the translation of the digging equipment 100 on the ground to move from one point in the building site to another, the movement of the arm 6 and of the tower 5 to position the digging tool 3 and the rotation of the winch 8 to lift or lower the tool 3 in the excavation. Such service functions are actuated by hydraulic actuators such as rotary motors or linear actuators installed on the base machine 2 and that always remain outside of the excavation.
The digging tool 3 generally consists of a cutter that is lowered into a pre-excavation of rectangular section. The pre-excavation is made with other digging equipment, like for example a bucket or a reverse boom excavator and, in order to avoid the walls crumbling, it is filled with stabilizing fluid that generally is a mud based on bentonite or polymers. The cutter consists of a prismatic frame 9 at the base of which two coil cutting devices for cutting the soil are arranged, like for example toothed drums 10, 11 rotating about parallel axes and actuated independently from hydraulic motors 12, 13. The hydraulic motors 12, 13 can be integrated in the toothed drums 10, 11, or they can be installed outside of them, fixed to the frame 9 and thus equipped with mechanical transmission for connecting to such toothed drums 10, 11.
The toothed drums 10, 11 break up (cut and crumble) the soil, ensuring the rectangular section of the excavation, and the debris broken up by the teeth in sufficiently small pieces is expelled from the excavation conveying it towards the surface through a submerged pump 14, also fixed to the frame 9 of the digging tool 3, which sucks it together with the stabilizing fluid with which the excavation is filled. The excavation fluid, therefore, can perform both a debris transportation function, and a stabilizing function of the excavation walls. Once it has reached the surface through the mud pipe of the pump 14, the excavation fluid is sent to suitable plants that take care of separating the solid part in suspension, whereas the liquid fraction is re-inserted into the excavation so as to always keep it full. In this way, the digging tool 3 advances removing soil up to the design depth, which in the most demanding applications can even exceed 200 meters.
In order to ensure that the excavation is sufficiently vertical, the cutter can be equipped with mobile flaps or shields 15 actuated by hydraulic cylinders 16. In this case, the frame 9 is very long (see FIG. 1). Alternatively, the frame 9 can be very compact in height if, for urban work or in low-height areas, the lowest possible bulk was required. The mobile shields 15, discharging a force against the walls of the excavation, can guide the digging direction so as to compensate for possible undesired deviations of the cutter.
The digging tool 3 thus performs multiple digging functions, including the following ones are essential: breaking up the soil through rotation of the cutting drums 10, 11, suction and transportation of the debris and correction of the digging direction. Such digging functions are actuated by hydraulic actuators, such as rotary motors or linear actuators, installed on the digging tool 3. These actuators are connected to the base machine 2 through hydraulic supply and discharge lines, also known as delivery and return lines, which supply the hydraulic power. The actuators of the digging apparatuses and the relative hydraulic lines are thus at least partially introduced into the excavation and immersed in the excavation fluid, and therefore are subjected to the hydrostatic pressure that, at the maximum depths reachable by the digging tools of this type, can be a few tens of bar. The digging tool, whilst being similar to that described up to now and thus equipped with at least one pair of toothed drums and a frame, can be used to break up the material and differ from the fact that it does not have a pump 14 installed. In this version the drums cut and break up the soil while a binding liquid is simultaneously inserted close to the wheels through a supply pipe coming from the outside. The action of the wheels pushes the soil mixed with the binder in a targeted manner above the frame. The tool can be guided with a rod or a “kelly”. In a further variant, the mixing tool can be guided by the frame through noses or flaps that stay in contact with the wall.
In the case in which the gaskets of the actuators of the digging tool are not perfectly efficient, or when the pipes and the relative fittings are not perfectly water-tight, or even due to problems deriving from incorrect compensation of the actuators (for example due to vibrations or pulsating phenomena induced by the digging, or due to a temperature variation), there can be penetration of the fluid of the excavation inside the hydraulic circuit of the digging equipment. The critical points of the hydraulic circuit, where the penetration can occur most easily, are the sliding gaskets, the pipe fittings, which can loosen, or possible cracks and cuts that can appear on worn pipes. The problem is particularly evident on the oil return lines towards the tank and on the draining lines of the rotary actuators, since in these lines the pressure inside the pipe can be lower than the hydrostatic pressure of the fluid in which they are immersed. In high-pressure supply lines (delivery lines), on the other hand, there is the reverse problem, since oil can leak towards the environment outside the pipe, with consequent dispersion of oil in the excavation.
In base machines according to the prior art, designed for applications with a hydrocutter, there is a single power engine installed inside the tower (generally endothermal, but which could also be electric) that supplies power to all of the hydraulic apparatuses both of the base machine, and of the digging tool. Since the flow rate of oil required for these apparatuses is very high, it would not be possible to supply it with a single pump and therefore multiple pumps are provided, each of which is dedicated just to a part of the apparatuses of the digging equipment. Very frequently, through a coupler, all of the pumps of the system receive mechanical power from the single power engine and transform it into hydraulic power. All the pumps suck the oil from a single tank installed inside the tower, in which the oil returns after having actuated the actuators connected to such pumps. In this case, all of the hydraulic system of the digging equipment, i.e. both of the base machine and of the digging tool, consists of a single circuit. Therefore, considering a defined volume of oil, it can be sucked by the tank through a first pump, be sent to a first actuator, return to the tank, be sucked from the tank through a second pump, be sent to a second actuator different from the first and return to the same tank. It is thus clear that the entry of polluting agents in the circuit causes the pollution of the entire circuit and can block or damage any actuator or other hydraulic component of the digging equipment.
In digging equipment according to the prior art the worker becomes aware of the pollution of the oil having occurred only after the malfunction or the blocking of a given actuator. In this situation the worker must interrupt all manoeuvres as soon as possible, just limiting himself to those strictly necessary to extract the tool from the excavation and position it in an area sufficiently far from the excavation to allow the building site workers to access the digging tool. The only way to block the spread of the contamination to other actuators is interrupting the manoeuvres and stop the pumps to block the circulation of the oil in the circuit. The damage and the consequent blocking of the functionalities of the actuators due to the pollution of the oil can be particularly serious if, during digging, with the tool immersed at great depth, the lifting apparatuses of the tool block. If, for example, one of the malfunctioning actuators is the winch combined with recovering the cutter from the excavation, it becomes impossible to extract the digging tool using only the base machine and it becomes necessary to use a second support machine, which is not always available in the building site. This means additional costs and very long down times.
In digging equipment according to the prior art sometimes a machine originally designed to perform only lifting works and therefore not specifically intended for being used in couple with a digging tool like a cutter is used as base machine. In these cases, the power of the motor installed on the base machine is usually not sufficient to ensure the simultaneous operation of the apparatuses of the base machine and of the cutter. In order to solve the problem, solutions are known in which a so-called additional external “power-pack” 43 (FIG. 1) is installed on the base machine with an additional hydraulic circuit. A final piece of digging equipment is thus obtained that comprises at least two power motors, where the first power motor, installed inside the casing of the tower, is intended to supply power to the apparatuses of the base machine, whereas the second power motor, included in the external “power-pack” 43, is intended to supply power to the apparatuses of the cutter. The external “power-pack” 43 is fixed, through suitable additional support frames, on the rear part of the rotary tower of the base machine. It is then positioned close to the ballast or replacing it, considering its substantial weight. The external “power-pack” 43 is very bulky, with typical values of its dimensions of the order of 3.5 meters×1.5 meters×2 meters. This positioning thus increases the tail radius of the base machine, i.e. the rear overhang with respect to the rotation axis of the tower on the tracked truck. The tail radius determines the area that is swept by the base machine during the rotation of the tower and, therefore, the area that must be kept free from objects or people that could be struck during rotation. This increase in the tail radius constitutes a limitation particularly in urban cutters, in other words in those cutters studied for use in built up areas where the maneuvering spaces are particularly small.
A further critical element of the solution that provides an external “power-pack” 43 is the fact that the addition of a second external power motor inevitably causes an increase in consumption with respect to a solution dedicated to digging applications, with a single motor of suitable power. Moreover, the external positioning of the “power-pack” 43 with respect to the casing of the tower causes an increase in the noise emitted, which must be limited particularly in urban applications. The increase in weight due to the mounting of the external “power-pack” 43 increases the pressure on the soil of the tracks and this results in a limit in movement on the building site. Finally, the accessibility of the “power-pack” 43 is awkward since it is typically installed at a high level and the frequent necessary maintenance can be dangerous and not very easy.